Event.observe(window, 'load', function() { addButtonHover(); });

function addButtonHover(){
  $$("li.top").each(function(element){element.onmouseover = function(){ Element.addClassName(element, "hover")}});
  $$("li.top").each(function(element){element.onmouseout = function(){ Element.removeClassName(element, "hover")}});
}